The invention relates generally to vulcanization compounds and relates more specifically to vulcanization compositions with reduced allergenic potential that include accelerator compositions for vulcanizing elastomeric articles.
There are two types of allergies associated with the use of elastomeric articles in the medical field: (a) Type I immediate hypersensitivity, IgE-mediated allergies; and (b) Type IV delayed hypersensitivity, cell-mediated allergies.
Type I hypersensitivity reactions are mediated by IgE immunoglobulin, and the effect is immediate. Generally, symptoms are evident within minutes of exposure to the allergen, and may include local urticaria, facial swelling, watery eyes, rhinitis, asthma, and in extremely rare occasions, anaphylactic shock. Type I allergies have been linked to the residual, extractable proteins present in natural rubber latex products.
Various technologies are available for reducing the extractable proteins in latex gloves, such as water leaching, chlorination, and the use of low-protein or deproteinized latex. However, healthcare personnel and patients who are allergic to natural rubber latex proteins are advised to use synthetic gloves. Commonly-used synthetic materials include polyisoprene, acrylonitrile-butadiene (nitrile), polychloroprene (Neoprene), polyurethane, and polyvinyl chloride.
As a result of the prevalence of Type I reactions in response to contact with natural rubber proteins, there has been a shift towards the use of synthetic latexes that do not contain natural rubber latex proteins, especially for use in making medical devices that come into contact with the skin. Taking cost and performance into consideration, synthetic latexes that are suitable for glove manufacture include nitrile latex and polyurethane latex for examination gloves, and polychloroprene latex and polyisoprene latex for surgical gloves. For surgical gloves, polyisoprene latex has typically been preferred over polychloroprene, even though it is more expensive, because it provides the gloves with properties that mimic those of natural rubber, particularly tensile strength, ultimate elongation, softness and comfortable feel.
However, Type IV allergic reactions can be caused by natural or synthetic elastomeric articles. Synthetic latexes can still cause allergic reactions due to the use of certain chemicals that may be found in the compounded latex. Type IV delayed hypersensitivity reactions are cell-mediated allergic responses to specific chemicals. Symptoms only become apparent about 48-96 hours after contact. Chemicals that may induce Type IV allergic responses include vulcanization accelerators such as thiurams, mercaptobenzothiazoles, dithiocarbamates, diphenylguanidines, and thioureas, which are used in the process of preparing the elastomeric articles. The U.S. Food and Drug Administration (FDA) acknowledges that thiazoles, thiurams, and carbamates in rubber products can induce Type IV allergic reactions in humans. “Guidance for Industry and FDA Reviewers/Staff: Premarket Notification [510(k)] Submissions for Testing for Skin Sensitization to Chemicals in Natural Rubber Products,” U.S. Department of Health and Human Services (1999). Hence, it is important to minimize the level of accelerators used so that the residual accelerator in the finished elastomeric article is very low.
Elastomeric articles are generally manufactured using a latex dipping process, which involves dipping molds or formers into a coagulant solution (usually aqueous calcium nitrate). After evaporating off the solvent, the coagulant-coated molds/formers are then dipped into compounded latex such that a film of coagulated rubber particles is deposited thereon. After gelling the latex film using heat, the wet-gelled latex film is leached in water and then dried and vulcanized in a hot air oven. During vulcanization the rubber molecules are chemically crosslinked.
Most commonly, the crosslinking agent is sulfur. However, sulfur alone is inefficient for forming crosslinks. Conventionally, sulfur has always been used in combination with vulcanization accelerators and activators.
Vulcanization accelerators are usually organic compounds that increase the rate and efficiency of sulfur crosslinking, while activators are compounds that increase the efficiency of the accelerators. Examples of accelerators used in latex compounding include thiurams, dithiocarbamates, mercaptobenzthiazole, diphenylguanidine, and thioureas. After vulcanization, depending on the amount of the accelerator used, some or most of the accelerators are chemically bonded to the rubber matrix, but some are unreacted and may remain as a residue in the finished elastomeric article.
Vulcanization activators used in latex compounding are usually metal oxides, such as zinc oxide, magnesium oxide, and lead oxide.
Various methods for minimizing or eliminating Type IV allergic reactions caused by vulcanization accelerators have been attempted, including crosslinking without the use of sulfur and vulcanization accelerators. Approaches include (a) crosslinking using gamma irradiation, (b) crosslinking using organic peroxides, (c) crosslinking using zinc oxide alone, via carboxyl-zinc ionic bonding, and (d) introducing functional groups into the polymer backbone that can form crosslinks after the product has been fabricated. Generally speaking, all of these approaches suffer from drawbacks. For example, approaches (a) and (b) result in products having poorer physical properties and poorer aging resistance than sulfur-vulcanized products.
Another approach is the use of safer accelerators. These are accelerators that have a lower allergenic potential. For example, a high-molecular weight accelerator that has low allergenic potential may be used, including, e.g., zinc dibenzyl dithiocarbamate (ZBEC), and zinc diisononyl dithiocarbamate (ZDNC). By virtue of their high molecular weights, these types of accelerators are more compatible with natural rubber and synthetic polyisoprene rubber, and therefore have a higher solubility in the rubber matrix. As a result, very little of the high-molecular weight accelerator would bloom to the rubber surface and come in contact with the user to cause a potential allergic reaction. For the same reason, very little of the high-molecular weight accelerator can be extracted from the rubber. ZDNC is preferred over ZBEC because it has a higher solubility in natural rubber (about 3% weight/weight), whereas the solubility of ZBEC is only about 0.5% weight/weight.
A further approach is to use combinations of fugitive accelerators, i.e., accelerators that are completely used up during vulcanization, leaving no residue in the product. Examples of such fugitive accelerators include dihydrocarbyl xanthogen polysulfides (which includes dialkyl xanthogen polysulfides) [short form “xanthogen polysulfides”] comprising diisopropyl xanthogen polysulfide (DIXP), diisopropyl xanthogen disulfide, diisopropyl xanthogen trisulfide, diisopropyl xanthogen tetrasulfide, diisopropyl xanthogen pentasulfide, diisoamyl xanthogen trisulfide, diisoamyl xanthogen tetrasulfide, diisoamyl xanthogen pentasulfide, diethyl xanthogen tetrasulfide, dibutyl xanthogen tetrasulfide, dibutyl xanthogen disulfide.
Using DIXP as a typical example, heating DIXP alone to high temperatures does not volatalize or decompose it completely to gaseous products. However, when DIXP is used together with sulfur and zinc oxide for crosslinking a diene containing polymer or rubber, it is consumed completely to form sulfur crosslinks, isopropanol and carbon disulfide as the major reaction products, leaving behind virtually no residue on the polymer or rubber since isopropanol and carbon disulfide would volatilize at the crosslinking/vulcanization temperatures. Since DIXP does not contain nitrogen in its chemical structure, it is also impossible to generate N-nitrosamines, which are associated with thiuram and dithiocarbamate accelerators. Additionally, certain nitrosamines are believed to be carcinogenic, and their formation should be avoided. However, DIXP alone does not accelerate sulfur crosslinking sufficiently to produce enough sulfur crosslinks to yield useful products, especially in polyisporene articles. The resulting articles have a tensile strength that is too low. Hence, DIXP has always been used in conjunction with other accelerators.
A variety of accelerator compositions have been disclosed in the prior art, some of which are discussed below.